What nvidia-drivers version? Is it nvidia-drivers-390.132-r2 (aka the one in ~unstable)? If so see bug #713546, temporary solution should be to either set USE=-libglvnd globally + update -U @world or have a look at the bug's attached patch, all it does is create a /etc/X11/xorg.conf.d configuration file that you could make yourself and preferably remove when no longer needed.
Edit: but I have no idea how well that may go with optimus, global -libglvnd may be a more favorable route on old drivers.

Also, I think that having nvidia-drivers built with libglvnd causes app-eselect/eselect-opengl not to be built and it had reverted to xorg-x11. After building nvidia-drivers with -libglvnd, eselect-opengl was built and I could select nvidia again._________________Michael Mounteney

The point of libglvnd is that it makes eselect-opengl obsolete by automatically choosing the right vendor for a screen/output, so lacking it is normal. And you can still manually pick the vendor with environment variables if need be (even has per-screen env vars)

However older nvidia drivers / ebuild have poor support, so I can't really recommend it on those right now.